These are the service line and emergency line. They attach to both the truck the semi trailer. The service air line is blue and will carry air that is controlled by the foot brake in the truck or the trailer hand valve.The service line is connected to a relay valve on the semi trailer and will apply pressure on the semi trailer brakes.he hand valve is never used in stopping, as it will cause the trailer to skid and possibly jackknife. The foot brake of the truck will also operate the brakes and prevent such events from occurring. The trailer air supply control is red 8-sided knob, which controls the tractor protection valve.
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